The invention relates to a novel process for the preparation of perdeuterated methacrylates with a high degree of deuteration.
In the preparation of optical fibres, materials based on organic polymers are gaining increasing importance, in addition to the inorganic glass materials. Polymethacrylates, in particular polymethyl methacrylate (PMMA), have proved to be very suitable due to their optical properties and material properties. It was found, however, that fibres based on PMMA still have light attenuation values which are too high for present-day requirements. Thus, for optical fibres of this material, these values are in the region of about 300 dB/km at a light wavelength of 650 nm. It has recently been found (Appl. Phys. Lett. 41, 802 (1982), and 42, 567 (1983)) that these attenuation values can be drastically reduced if the corresponding deuterated polymer is used in place of PMMA for the preparation of the fibres. Thus, with optical fibres of perdeuterated polymethyl methacrylate, which is accessible by polymerisation of deuterated methyl methacrylate (MMA-d.sub.8), attenuation values of about 20 dB/km can be realised. In this case, however, an important point is that the perdeuterated monomer, as well as the polymer, must be as pure and as highly deuterated as possible. In fact, it has been found that there is a direct relationship between the degree of deuteration of the polymer and the light attenuation value of the fibre, with the result that the light attenuation is very substantially reduced only when the degree of deuteration is as high as possible.
Thus, it should be possible to synthesise MMA-d.sub.8, used as the starting material for the preparation of perdeuterated PMMA, in the most highly deuterated form.
Synthesis processes for perdeuterated methyl methacrylate are known, for example from U.S. Pat. No. 4,138,194. In this process, in accordance with the following equation ##STR1## perdeuterated acetone (hexadeuteroacetone, acetone-d.sub.6) is first reacted with hydrogen cyanide to give the corresponding cyanohydrin. The latter can then be converted, by treatment with concentrated sulfuric acid and perdeuterated methanol, whereby the cyanohydrin is dehydrated and its nitrile group is hydrolysed and esterified, directly into MMA-d.sub.8.
As shown by the applicant's own work and by literature data (Makromol. Chem. 182, 2502 (1981)), however, the degree of deuteration in the ketone part of the resulting MMA-d.sub.8 is, with this synthesis route, due to a partial H/D exchange, up to 10% lower than in the acetone-d.sub.6 used as the starting material. This loss of deuterium due to H/D exchange can be avoided only if DCN, which, however, is extremely expensive, is used in the cyanohydrin synthesis step in place of HCN.